1. Field of the Invention
The present invention relates to a toner, a two-component developer, a developing device, and an image forming apparatus.
2. Description of the Related Art
A toner is used to visualize a latent image in various image forming processes, and one known example thereof is an electrophotographic image forming process.
An electrophotographic image forming apparatus has been widely used as copiers so far, and in recent days, also as printers, facsimile machines, and the like equipment along with spread of computers since the electrophotographic image forming apparatus operates excellently as output units for computer images created by computers. In a general electrophotographic image forming apparatus, a desired image is formed on a recording medium through a charging step, an exposing step, a developing step, a transferring step, a fixing step, and a cleaning step. In the charging step, a photosensitive layer on a surface of a photoreceptor drum is homogeneously charged. In the exposing step, the charged surface of the photoreceptor is irradiated with signal light corresponding to an original image so that an electrostatic latent image is formed. In the developing step, an electrophotographic toner (hereinafter referred to simply as “toner”) is supplied to the electrostatic latent image on the surface of the photoreceptor drum so that the electrostatic latent image is formed into a visualized image. In the transferring step, the visualized image on the surface of the photoreceptor is transferred onto a recording medium such as paper or OHP sheet. In the fixing step, the visualized image is fixed onto the recording medium by heat, pressure, etc. In the cleaning step, a toner and other matters remaining on the surface of the photoreceptor from which the visualized image has been transferred, are removed by a cleaning blade, and the surface of the photoreceptor is thus cleaned. Note that the visualized image may be transferred onto the recording medium by way of an intermediate transfer medium.
In recent years, there has been a tendency to make a toner particle smaller in order to meet the demand of higher image quality. However, there arises a problem of fog caused on a non-image part and in particular, the fog generated after toner replenishment is problematic. This problem of fog caused on a non-image part arises due to a fact that the smaller diameter of toner particles increases a charge amount in bulk while a charge amount on each toner particle (especially a charge amount on a small toner particle) is so small that the charge on the toner particle is not sufficiently controlled, which makes charge amount distribution unstable. In order to improve such instability of the charge amount distribution, the following methods have been proposed.
Japanese Unexamined Patent Publication JP-A 7-199520 (1995) discloses a technique of mixing at least two groups of toner particles that are different in average particle size and each contain at least one internal additive (magnetic powder, charge control agent, and lubricant) of which particles are different in size from group to group, to thereby minimize a difference in charge amount among particles depending on their sizes, which difference is attributable to an increase in breadth of toner particle size distribution, thus allowing for stable image quality having no fog on the non-image part.
Japanese Unexamined Parent Publication JP-A 9-114127 (1997) discloses a technique of balancing high image quality with developability (proper density with fog prevention) by setting specific conditions on a volume average particle size, a content of color material, and a weight of a toner to be developed, with the respect to the toner having a volume average particle size of 3.0 μm to 9.0 μm, and in order to form a higher-quality image, it discloses setting of the particle size distribution to D50P/D84P≦1.45 or 1.25≦Dn50/Dn25≦1.50.
Japanese Unexamined Patent Publication JP-A 2005-234410 discloses, in a toner containing at least a polyester resin, a colorant, a release agent, hydrophobic silica and titanium oxide, a technique of dispersing a wax component into the resin to be island shaped; setting quantity of wax present in the vicinity of toner particle surface to 2% by weight to 10% by weight of all the components contained in the toner which figure is a measurement value according to the all-reflection infrared spectroscopy; setting the volume average particle size (Dv) of the toner to 4.0 μm to 6.0 μm; and setting the shape factor SF-1 of the toner particles to 140 to 200, to thereby maintain favorable fluidity of the toner and form a high-quality image having no fog.
In the invention disclosed in JP-A 7-199520, the size difference between smaller particles and larger particles of the charge control agent in the particle size distribution is effective for the long-term charging stability while it is difficult to sufficiently charge the toner of small particles immediately after toner replenishment.
In the invention disclosed in JP-A 9-114127, the particle size distribution is set to D50P/D84P≦1.45 or 1.25≦Dn50/Dn25≦1.50. In a condition with such particle size distribution, a content of fine toner particles having a volume average particle size of 4 μm or less is not sufficient and it is therefore impossible to form an image having sufficiently high definition and high resolution.
In the invention disclosed in JP-A 2005-234410, the quantity of wax present in the vicinity of toner particle surface is high, to be specific, 2% by weight to 10% by weight of all the components contained in the toner, leading to lower fluidity and incapability of controlling charge on each toner particle, with the result that the fog cannot be suppressed on the non-image part. Moreover, only the volume average particle size is specified, and it is therefore not possible to obtain an image having sufficiently high definition and high resolution. In the region where SF-1 is large, a transferring property is a major issue.